Sickle cell disease is a debilitating inherited hemoglobin disorder that is the most common single-gene disease in the world. Hematopoietic stem cell transplantation is the only curative therapy for SCD; however toxic myeloablative conditioning regimens and barriers to allotransplantation have limited its use to children with major complications and HLA matched donors. New myelosuppressive/immunosuppressive transplant strategies are emerging to reduce morbidity and mortality and to make cell transplantation available to a larger number of patients by intentionally inducing mixed hematopoietic chimerism. However, these protocols raise significant issues that can be best addressed in a preclinical model. Using a murine model of sickle cell disease that expresses exclusively human sickle hemoglobin we have defined a non-myeloablative transplant protocol that induces mixed hematopoietic chimerism and tolerance to MHC disparate donors while correcting hematologic and pathologic manifestations of the disease. In Aim 1, we will extend these studies to determine the levels of donor chimerism that provide hematologic and/or physiologic cure of sickle cell disease. We will determine if very low levels of stem cell chimerism can induce and maintain allogeneic tolerance, and whether genetically modified cell populations can be expanded to provide a permanently corrective mixed chimeric state. Sickle cell disease is now recognized as having complex inflammatory interactions between multiple cell types that lead to pathological outcomes. These interactions may also be responsible for the increased rate of rejection found in stem cell transplantation for sickle cell disease. In Aim 2 we will investigate inflammatory and immunological mechanisms involved in this rejection process. We will investigate co-stimulation blockade resistant rejection, immune effector populations, adhesion molecules and cytokines for their involvement and contribution to allogeneic rejection. These aims provide a comprehensive systematic approach to studying the relationship between mixed chimerism and sickle pathophysiology and the enhanced rejection rate found in transplantation for sickle cell disease. Both Aims address basic mechanisms of transplantation tolerance and rejection as well as providing the critical preclinical data that are required for the design of future non-myeloablative transplants protocols. [unreadable] [unreadable] [unreadable]